Wednesday, September 30, 2015

When to Replace Anchor Chain

A Potential Bad Link
I noticed this summer that the anchor chain was flaking rust when anchoring. I've never had an anchor chain rust enough to flake and was concerned. I started investigating engineering rules for safe chain to determine when we should replace it. Looking on the boating forums was not helpful. Many people just say "It was rusty, so I replaced it." I couldn't find an engineering analysis of chain and corrosion guidelines. Weren't there guidelines? This is a situation for facts, not opinions.

I checked the existing anchor chain and found it stamped "G4", whose links are .394 in diameter with a 1.22 in inside length (called pitch in some documents). This is 10mm or 3/8 in chain and fits our chain gypsy.

After much searching I found that the Peerless Chain web site has a nice document ( that tells the minimum thickness due to wear for Grade 80 chain (the same dimensions as our G4 3/8 chain). The link thickness when new is .394 in and should be replaced when it is .342 in, or an erosion of 13%. A ship web site (see link below) mentioned that a reduction of 12 percent is allowed before replacing a vessel's chain. .394 * .88 = .3467

To put these figures in perspective, the next smaller chain is 5/16, which has a diameter of .329 in and has a Working Load Limit (WLL) of 3900 lb. A new G4 3/8 chain would have a WLL of 5400 lb. The WLL is 1/3 of the minimum breaking strength of the chain, providing a safety margin. (Note: G70 and above chain have a WLL that's 1/4 of the breaking strength because those chains are certified for overhead lifting of loads.)

These pieces of information gave me a start on deciding if our chain was safe. I decided that .350 was my threshold for replacing the chain (11% erosion). I then looked for the most rusty links I could find and sure enough, the first link I tried was .350 thickness. The link was cleaned with a file to clean, shiny metal to prevent false readings. Our chain needed to be replaced.
Hmmm - 0.350

I checked around and the best price was at Bacon Sails in Annapolis. An order for 200ft was placed and they had it in less than a week. Wire ties were used to mark the chain every 25 ft, clipping the tails so that they are long enough to see the color. LUX was out of the water at that point, so I simply backed my car under the boat and loaded the chain right into the chain locker. Now we can sleep on the hook without wondering if the anchor chain will fail.

We are also planning to wash the chain with fresh water after anchoring.

Colorful Wire Ties for Markers 
Loading the Chain

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